1. Field of the Invention
The present invention relates to a minute pitch measuring method and apparatus, and more particularly to a minute pitch measuring method and apparatus for measuring a track pitch of an optical disk or the like.
2. Description of the Related Art
Conventionally, the minute pitch or track pitch of an optical disk is measured by using an optical microscope. For example, the track pitch is measured by applying a laser beam to the optical disk, and observing its diffracted light (see JP-A-8-329533 (FIG. 12, page 2)).
FIG. 1 shows a schematic constitution of a conventional track pitch measuring apparatus and method. Referring to FIG. 1, the track pitch measuring method will be described below.
As shown in FIG. 1, a conventional track pitch measuring apparatus 50 for optical disk measures a track pitch Tp for the track T of an optical disk 52 by applying a laser beam from a laser generating part 51 for generating the laser beam to the optical disk 52, employing a gas laser or semiconductor laser represented by a helium-neon laser, and observing its diffracted light on a screen 53.
A beam diameter of the laser beam is as large as about 0.3 to 1 mm, in which the track pitch is measured by applying a laser beam to the optical disk 52, and performing an averaging process over its irradiated area.
Herein, assuming that the interval between the zeroth order diffracted light L0 and the first order diffracted lights L+, L− is d, and the interval between the optical disk 52 and the screen 53 is L, the track pitch Tp is represented such as,TP=(λ/d)×L
By the way, in the case of an optical microscope, a resolution limit is decided by the laser wavelength λ and the numerical aperture NA of an objective lens. At present, a high resolution microscope having a ultraviolet laser (λ=266 nm) and an objective lens of NA 0.9 is commercially available, in which the resolution limit is about 0.15 μm.
If the optical disk has a higher density with a narrower track pitch (e.g., 0.3 μm or less) in the future, there is a problem that the track pitch is not measured accurately, because the above optical microscope is also close to the resolution limit (0.15 μm). Also, since the track pitch is measured in each observation area through the sequential feed with a stepping motor, it takes a considerable measurement time to measure the entire area of disk.